Trends in harvests and pup numbers of the South African fur seal: implications for management

Wickens, P. A.; David, J. H. M.; Shelton, Peter A.; Field, J. G.

doi:10.2989/025776191784287745

Cited by 30 publications

(34 citation statements)

References 2 publications

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“…Subscribers to this point of view believe that increases in the size of the seal population negatively affect the catches of the fisheries. Their concerns have intensified in recent decades, because the fur seal population has increased at about 3% annually since 1971 (Wickens et al, 1991), despite substantial annual harvests of seals in Namibia (suspended in South Africa after 1990; Wickens and York, 1997). The Cape fur seals' total population size in Namibia was estimated at just over half a million individuals during a survey in December 2001 (Marine and Coastal Management, MCM, South Africa, unpublished data).…”

Section: Introductionmentioning

confidence: 98%

A refined fish consumption model for lactating Cape fur seals (Arctocephalus pusillus pusillus), based on scat analyses

Mecenero

Kirkman

Roux³

2006

ICES Journal of Marine Science

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Mecenero, S., Kirkman, S. P., and Roux, J-P. 2006. A refined fish consumption model for lactating Cape fur seals (Arctocephalus pusillus pusillus), based on scat analyses. e ICES Journal of Marine Science, 63: 1551e1566.A refined fish consumption model for lactating Cape fur seals in Namibia during the eightmonth lactation period, which allows for spatio-temporal variation in the diet as determined by scat analyses, has been developed. Previous estimates of prey consumption by Cape fur seals have been based mostly on coarse diet composition models. Sensitivity analyses showed that the energetic requirement and mass of lactating females (bioenergetic variables), as well as the energetic density of prey (diet variable), contributed most to the uncertainty in consumption estimates. Uncertainty in the remaining input variables had minimal effects on the estimates of food consumption. The consumption of commercial prey (horse mackerel, hake and pelagic fish) was greatest by the colony at Cape Cross. The model estimated that a female of average mass 55 kg ingested, on average, 11% of her body mass per day. This model is easily applied to other age/sex classes of the seal population. It permits improvement of the estimates of prey consumption by seals, which are useful for assessing levels of competitive interactions between seals and fisheries or other predators, or the impacts of seals on prey species.

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Section: Introductionmentioning

confidence: 98%

A refined fish consumption model for lactating Cape fur seals (Arctocephalus pusillus pusillus), based on scat analyses

Mecenero

Kirkman

Roux³

2006

ICES Journal of Marine Science

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“…Under controlled harvesting from the beginning of the 20th century, the population increased from <100,000 to around 2 million individuals overall between 1920 and the late-1990s, distributed throughout the SB and NB (Butterworth et al 1995). Sealing stopped in South Africa in 1990 (Wickens et al 1991), but is still practiced in Namibia, at three mainland colonies (Kirkman and Lavigne 2010). Since the early 1970s, abundance trends have been estimated from series of aerial, photographic censuses of pups at breeding colonies in both the SB and NB.…”

Section: Jellyfishes In the Benguelamentioning

confidence: 99%

Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

Roux¹,

Lingen²,

Gibbons³

et al. 2013

BMS

123

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Changes in two contrasting ecosystems of the Benguela upwelling region, one dominated at mid-trophic level by jellyfishes (Namibia, northern Benguela ecosystem, where small pelagic fish abundance has been severely depleted) and one still dominated by small pelagic fishes (South Africa, southern Benguela) were compared in an effort to determine ecosystem trajectories under different exploitation regimes. The role of small pelagic fishes (clupeoids) was highlighted in the context of their importance in maintaining interactions in marine ecosystems. In particular, we examined trophic cascades and possible irreversible changes that promote the proliferation of jellyfishes in marine systems. We found that the presence of large populations of small pelagic fishes has a fundamental role in preserving beneficial trophic interactions in these marine ecosystems. The implications of trophic cascades, such as those observed in the northern Benguela, for ecosystem-based management were apparent. In addition, this comparison provides contrasting case studies to inform the development of management scenarios that avoid ecosystem shifts that affect predators and reduce the value of fisheries production.

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“…This assumption is known as stationarity. However, many pinniped species, including the Cape fur seal, have been subject to periodic harvest (Wickens et al 1991) and compete with a growing fishing industry (Wickens et al 1992). When the effects of anthropogenic activity on pinniped populations are taken into consideration, the above assumption must be questioned.…”

Section: Body Weight and Agementioning

confidence: 99%

Gross and microscopic visceral anatomy of the male Cape fur seal, Arctocephalus pusillus pusillus (Pinnipedia: Otariidae), with reference to organ size and growth

Stewardson

Hemsley

Meyer³

et al. 1999

Journal of Anatomy

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The gross and microscopic anatomy of the Cape fur seal heart, lung, liver, spleen, stomach, intestine and kidneys (n l 31 seals) is described. Absolute and relative size of organs from 30 male seals are presented, with histological examination conducted on 7 animals. The relationship between log body weight, log organ weight and age was investigated using linear regression. Twenty five animals were of known age, while 6 were aged from counts of incremental lines observed in the dentine of tooth sections. For the range of ages represented in this study, body weight changes were accurately described by the exponential growth equation, weight l w o r t , with body weight increasing by 23 % per annum until at least 9-10 y of age. Organ weight increased at a rate of between 25 % and 33 % per annum until at least 9-10 y of age, with the exception of the intestines, where exponential increase appeared to have ceased by about 7 y. The relationship between body weight and organ weight was investigated using logarithmic transformations of the allometric equation, y l ax b , where the exponent b is 1 if organ weight is proportional to body weight. Most organs increased in proportion to the body. However, the heart, liver and spleen had exponents b 1, suggesting that these organs increased at a faster rate than the body. The basic anatomical features of the viscera were similar to those of other pinnipeds, with some exceptions, including the arrangement of the multilobed lung and liver. Apart from the large liver and kidneys, relative size of the organs did not differ greatly from similar sized terrestrial carnivores. The histological features of the organs were generally consistent with those previously described for this species and other otariids. The heart, as in other pinnipeds, was unlike that of cetacea in not having unusually thick endocardium or prominent Purkinje cells. Notable histological features of the lungs included prominent fibrous septa, prominent smooth muscle bundles, cartilage extending to the level of the alveolar sacs and ample lymphoid tissue. The spleen had a thick capsule, well developed trabeculae and plentiful plasma cells. Abundant parietal cells were present in the fundic glands and lymphoid follicles were present in the gastric lamina propria, particularly in the pyloric region. Small intestinal villi were very long but this could have resulted from underlying chronic inflammation. Lymphoid follicles were prominent in the colon. The kidney reniculi each had a complete cortex, medulla and calyx, but a sportaperi medullaris musculosa was not identified.

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Trends in harvests and pup numbers of the South African fur seal: implications for management

Cited by 30 publications

References 2 publications

A refined fish consumption model for lactating Cape fur seals (Arctocephalus pusillus pusillus), based on scat analyses

A refined fish consumption model for lactating Cape fur seals (Arctocephalus pusillus pusillus), based on scat analyses

Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

Gross and microscopic visceral anatomy of the male Cape fur seal, Arctocephalus pusillus pusillus (Pinnipedia: Otariidae), with reference to organ size and growth

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